Shear-induced deformations of polyamide microcapsules

The application of microcapsules for technical, cosmetic and pharmaceutical purposes has attracted increased interest in recent years. The design of n ew capsule types requires a profound knowledge of their mechanical properti es. Rheological studies provide interesting information on intrinsic membra ne properties and this information can be used to avoid premature release o f encapsulated compounds due to the action of external mechanical forces (s tirring, swallowing, spreading). In this publication we report a systematic study of polyamide microcapsules. These particles were synthesized by reac ting 4-aminomethyl-1,8-diaminooctane and sebacoyl dichloride at the interfa ce between silicone oil and water. Two different experiments were performed to get information on the mechanical properties of the capsule walls. Firs t of all, we used an optical rheometer (rheoscope) to observe the capsule d eformation and orientation in shear flow. The polymerization kinetics, rela xation properties, the regime of linear-viscoelastic behavior and the shear modulus of the flat membranes were independently measured in an interfacia l rheometer. Both experiments gave complementary results. It turned out tha t the two-dimensional elongational modulus was about 3-4 times larger than the shear modulus. This result is in fairly good agreement with a theoretic al model recently proposed by Barthes-Biesel. Due to the simple synthesis a nd well-defined structure, polyamide microcapsules can also serve as simple model systems to understand the complicated flow properties of red blood c ells.